The present invention relates to a solder resist composition used for a printed-circuit board 2. Description of the Prior Art
In the manufacture of a printed-circuit board, a patterned protective film formed of a solder resist is formed on a printed-circuit board in order to protect the electric circuit and to prevent a solder from being attached to an undesired portion in the step of soldering an electric part to the printed-circuit board.
It was customary to employ a screen printing method for forming a patterned layer of a solder resist. With recent progress in the high densification of the circuit in the printed-circuit board, a photo-setting acrylate or methacrylate solder resist capable of patterning by a photoetching method has been developed in place of the conventional screen printing solder resist The newly developed solder resist permits forming a fine pattern of high dimensional accuracy and high resolution at a high density. In the case of using the newly developed solder resist, the surface of a printed-circuit substrate is coated with the solder resist, followed by selectively exposing the solder resist film in a desired pattern to light using a negative mask so as to photoset the exposed portion. Then, the non-exposed portion is dissolved in a developing liquid and removed so as to form a solder resist pattern The photo-setting acrylate resin certainly exhibits excellent properties when used as a solder resist, but is not sufficiently satisfactory in, for example, electrical properties, solder heat resistance and humidity resistance.
A solder resist intended to overcome the above-noted difficulties is proposed in Japanese Patent Disclosure 60-208377. The proposed resist comprises the reaction product between a phenol novolak type epoxy resin and an unsaturated monobasic acid, the reaction product between a cresol novolak type epoxy resin and an unsaturated monobasic acid, a photopolymerization initiator, an amine type curing agent and an organic solvent. However, the phenol novolak type epoxy resin is lower in solubility in an organic solvent than the cresol novolak type epoxy resin. The reaction product between the phenol novolak type epoxy resin and an unsaturated monobasic acid is still lower in solubility in an organic solvent As a result, the kind of solvent used as a developing liquid of the solder resist is much restricted. To be more specific, a nonflammable developing liquid such as 1,1,1-trichloroethane, trichloroethylene or tetrachloroethane is generally used in view of the safety in the manufacturing process of a printed-circuit board. Since the solder resist disclosed in the prior art quoted above is low in solubility in such a nonflammable developing liquid, the development takes a long time. The reaction product between the cresol novolak type epoxy resin and an unsaturated monobasic acid is also low in its solubility, though not so low as the phenol-novolak resin, and not satisfactory in its thermosetting properties.
Further, Japanese Patent Disclosure No. 62-4390 discloses a curable resin composition for a solder resist ink. The composition comprises a solution containing the reaction product of a phenol novolak type epoxy resin or a cresol novolak type epoxy resin with an unsaturated monobasic acid, dissolved in an organic solvent and/or photopolymerizable polyfunctional vinyl monomer. A photopolymerization initiator and an amine during agent are added to the solution. The resin composition will be accompanied by disadvantages similar to those of the Japanese Patent Disclosure 60-208377.
Japanese Patent Publication No. 60-32360 discloses a solder resist composition comprising the reaction product of an aromatic polyepoxide with an ethylenically unsaturated carboxylic acid, an inorganic filler, a photopolymerization initiator, and an organic solvent. The composition is insufficient in electrical properties and solder heat resistance, in particular.